1. Technical Field
The present disclosure relates to an improved electrolytic device. In particular, according to the present disclosure, a high-quality amorphous metal or metalloid chalcogenide film obtained by a relatively high throughput/low-temperature solution-deposition method is used as the active electrolytic layer in the solid-state electrolytic device. The present disclosure also relates to a process for fabricating the solid-state electrolytic device
2. Background Art
In solid-state electrolyte devices, the solid-state electrolyte material generally consists of a Ag-, Cu-, Zn- or Li-doped amorphous chalcogenide (most typically, GeS2, GeSe2, As2S3, As2Se3), which acts like an excellent conductor of ions (i.e., Ag+, Cu+, Zn2+, Li+, etc). The amorphous chalcogenide may be of a binary composition (i.e., GeS2-x, GeSe2-x, As2S3, As2Se3) or may contain three or more elements (i.e., Ge1-xSnxS2-ySey, GeSbxSy, AS2-xSbxS3-ySey, GeSe2-yTey, etc.). Doping of the amorphous chalcogenide is obtained by either co-deposition or by electrical/thermal or UV diffusion of the metal or metalloid into the pre-deposited amorphous chalcogenide. UV diffusion is normally the preferred technique and results in a system that has a saturated and uniform concentration of the dopant in the material (which depends on the stoichiometry of the starting material). Both the undoped and doped chalcogenides have very high resistance (typically >1 gigaohms for a 30×30×30 nm region).
If such a doped amorphous chalcogenide material is sandwiched between two metals, one of which is reactive (i.e., containing the dopant Ag, Cu, Zn, Li; henceforth referred to as the anode) and the other one of which is an inert material (e.g., W, TiN, TaN, Al, Ni, etc.; henceforth referred to as the cathode), then the following electrical effects are achieved:                (a) On the application of a small positive bias (bias being applied to the anode), it is believed that ions diffuse from the anode and the solid electrolyte material towards the cathode and form a conducting “metallic” filament that starts at the cathode and builds up towards the anode. This happens as long as the applied bias is greater than the “threshold” voltage for formation (which depends on the material and the bottom electrode and is generally between 0 and 1.0 V). When the filament is fully formed, it will result in a short between the 2 electrodes. This results in a very low resistance state (typically <1 Mohm). This conducting filament stays for a period even after the applied bias has been removed. The formed filament, though nominally permanent, tends to diffuse back into the electrolyte (with no applied bias), causing the on-state resistivity to increase over time (faster at elevated temperature). Typically the ON resistance is a function of the steady-state current during programming: Ron=Vth/Ion, where Vth is the electrodeposition threshold (typically lower than the threshold voltage for formation). Typical programming times are of the order of 50-100 ns or faster, but may be much slower depending on various factors including the method of deposition of the amorphous chalcogenide.        (b) When a negative bias is applied to the anode (and a conducting filament already exists), then ions move out of the conducting filament back into the solid electrolyte and eventually into the anode. A break in the metallic filament can result in high resistance.        Typically for times <100 ns, the entire metallic filament is erased and an ultra-high resistance is obtained.        
These device characteristics are illustrated in FIG. 1. FIG. 1 illustrates electrolytic device characteristics for an Ag—Ge—S device. At a slight positive voltage (Va=0.2 V), the device switches to the low resistance state. At Vb=−0.2V, the device switches to the high resistance state. Va (turn-on voltage) and Vb (turn-off voltage) range from 0 to +/−1V and depend on the material and the cathode. The ON resistance of the solid electrolyte memory is a function of the program current (i.e., Ron=Vth/Ion). The OFF resistance is a function of the resistivity of the solid electrolyte and of the double layer at the interface. The device turns on by forming a thin metallic bridge between the inert and oxidizable electrode. Vth is the electrodeposition threshold and Ion is the on-current that is used for programming (see Kozicki et al, Programmable Metallization Cell Memory based on Ag—Ge—S and Cu—Ge—S, NVMTS 2005, pp. 83-9).
The following are some potential applications for a solid electrolyte material:
(a) Use as a memory material—where the low and high resistance states can be labeled as 1 and 0 respectively (see Kozicki et al., IEEE Trans. Nanotech. 4, 331(2005) and Terabe et al, J. Appl. Phys. 91, 10110(2002).
(b) As a diode material-especially for high current density memory elements.
Typically, the amorphous chalcogenide material (either doped or pre-doped) is deposited using a vacuum-based technique such as sputtering or thermal evaporation. Such techniques are relatively costly and time-consuming, since they rely on achieving a high-vacuum environment in a confined space prior to the deposition. In addition, compositional control may be difficult to achieve due to effects such as preferential sputtering in composite targets, the need to balance evaporation rates for multiple evaporation sources and the added difficulty of vacuum-depositing sulfur compounds because of the high vapor pressure of sulfur. Finally, deposition on complex surfaces (i.e., those containing vias and trenches) can be problematic for directional sputtering techniques.
Therefore, it would be highly desirable to develop alternative methods for depositing the amorphous chalcogenide active layer for an electrolytic device.